The primary difference and most noticable will be the overall look of the different stones. Secondly marble is a much softer stone and will scratch and etch. Marble is also more likely to have integrety issues during fabrication. It tends to break easily. Also it is not recommended that you use marble in the kitchen, where as granite can be used in the kitchen.

 What is etching?

Etching is the damage that occurs to the surface of your countertop. Sometimes this is apparent by feeling the stone and noticing a much rougher area. Other times when it can not be felt the stone will just seem duller in certain areas. This occurance is can caused by lemons, alcohols, and also by some cleaning products.